1. Field of the Invention
The invention relates to lamps and lighting systems for use underwater by divers. More particularly, the invention relates to an HID lamp with integral ballast and underwater lighting systems incorporating the same.
2. Brief Description of the Prior Art
Underwater exploration, or exploration in other low ambient light environments, is virtually impossible without the aid of some type of artificial lighting system. Even under broad daylight, when diving beyond a certain depth, the natural light from the sun is severely affected by the water. In addition to loss of light intensity, the water produces spectral changes in the light to the extent that color is not readily recognizable and the view underwater appears to be only bluish black and white. Moreover, even at relatively shallow depths, artificial lighting is necessary to see objects in shadows or in crevices. Exploration of caves, shipwrecks, or comparable very dark or harsh environments, is impossible without bright artificial lighting systems.
The simplest lighting systems utilize ordinary incandescent lamps powered by rechargeable batteries. Ordinary incandescent lamps are inefficient and produce a limited spectrum which is unsuitable for photography, particularly under water. Halogen lamps provide a much higher intensity than ordinary incandescent lamps and also provide a balanced spectrum which can be used with certain types of film to accurately capture colors in underwater photographs. For example, many halogen lamps are balanced to a color temperature of 3200° K., and some film emulsions are designed to be used with illumination with this spectrum. Filters are also available for use with daylight (6500° K.) balanced emulsions and 3200° K. light sources.
Although the halogen lamps are an improvement over ordinary incandescent lamps, they share some of the disadvantages of ordinary incandescent lamps and have some disadvantages of their own. Both kinds of lamps rely on the heating of a filament by an electric current passing through the filament. In order to produce more light output and a higher color temperature, more current must be provided to the filament. This requires either a larger battery or results in a shorter “burn life”. Since divers and explorers are burdened with enough equipment to begin with, a large battery pack is certainly undesirable. Filament lamps also have the disadvantage that the filament is easily damaged by thermal or mechanical shock.
A new type of lamp referred to as a high intensity discharge (HID) lamp is disclosed in U.S. Pat. No. 5,144,201 (the complete disclosure of which is hereby incorporated by reference herein) and is generally available from Welch Allyn, Inc. (Skaneateles Falls, N.Y.). The lamp contains an anode and a cathode and a mixture of mercury, argon and other chemicals. The anode and the cathode are coupled to a ballast having a DC power input. When a DC voltage (9-16 VDC) is applied to the power input of the ballast, the ballast begins a start-up sequence. The ballast first produces a series of high voltage (25 KV) high frequency (33 KHZ) pulses that ionize the gases inside the lamp. During this sequence the ballast monitors the resistance of the lamp. When the gases have been sufficiently excited, an arc is struck across the anode and cathode. After the arc is struck, the ballast applies a reduced DC voltage to the anode and cathode of approximately 60 VDC. The ballast continuously monitors the resistance of the lamp and controls the current to the lamp in order to maintain the arc and prevent overdriving, see U.S. Pat. No. 5,381,076 (the complete disclosure of which is hereby incorporated by reference herein). The color of the light produced by the HID lamp is determined by the mix of material (compounds and/or gases) contained in the lamp and the extent to which they are excited by the continuing current. Typically, the desired color temperature is in the range of 4700-6500° K.
The HID lamps provided by Welch Allyn and others are not particularly designed for use under water. Many manufacturers intend that these lamps be used in automotive applications and in image projection applications such as projection television. For a variety of reasons, Welch Allyn recommends that the lamp and ballast be located apart from each other. In most applications, this does not present a problem. However, in an underwater lighting system, location of the ballast apart from the lamp can be problematic. The typical underwater lighting system includes a battery pack which is coupled by a cable to a lamp assembly which may be hand held in smaller sizes of lighting systems. If the ballast is not located adjacent to the lamp assembly, it must be located adjacent to the battery pack. The battery pack is typically strapped to the diver's torso, arm or leg. In order for the lamp assembly to be freely positionable, the cable connecting the lamp assembly to the battery pack must be sufficiently long. It has been discovered, however, that if the cable length from the ballast to the lamp assembly is more than approximately 18 inches, the lamp may not reliably startup.